Intercarrier noise suppressor



Nov. 23, 1937. R. D. BROWN. JR

INTERCARRIER NOISE SUPPRESSOR {filed May 8, 1935 a Patented Nov. 23,1937 UNITED STATES PATENT OFFICE mrnncanama NOISE SUPPRESSOB ReynoldsD.Brown, In, Philadelphia, Pa... as-

signor to l ny, Philadelphia, Pennsylvania" Philadelphia Storage BatteryCom- Pa., a corporation of Application May a, 1935, sci-a1 No. 20,411 InGreat Britain May 14,1934- 4 Claims. (01. 250-20) type above mentionedembodying the features of This invention relates to radio receivers andhas for'its principal object the provision of a novel inter-carriernoise suppressor; The invention is useful particularly'in a system suchas 0 for broadening the fidelity curve or response characteristic of thereceiver to give a desired,

substantially rectangular response curve, and to eliminate sidebandcutting; and also embodying r a tuning indicator circuit connected tothe main circuit so as to derive a portion of the incoming energytherefrom. The present invention is very well adaptedfor use inconjunction with such a system and it will, therefore, be describedherein 'with reference to this specific adaptation, al- 0 though it willbe understood that the invention is not thus limited but is capable ofapplication in any desired and suitable manner.

The main feature of the invention comprises the use of a resistor whichhas developed thereacross a unidirectional potential which varies inaccordance with the various signal strengths received during the tuningof the, receiver. This variable potential is applied to an amplificationcontrol element in one of ,the circuits of the 0 audio frequencyamplifier of the main circuit, thus placing [a variable bias on the saidcontrol element and effecting gain control of the audiov frequencyamplifier. There is thus provided an inter-carrier suppressor whichallows functioning 3 of the receiver only when it is properly tunedtothecarrier frequency. In the absence of this suppressor, it would bedifllcult to properly tune the receiver by ear as the receiver wouldrespond to the same extent even when tuned consider- 40 ably oil thecarrier frequency. The resistor above mentioned has developed across ita nega tive biasing potential which is of such value as I to cut off theaudio frequency amplifier excepting when the receiver is tuned to thecarrier.

# A further feature of the invention resides in its-particularadaptation for use in connection with automatic volume control. By thisfeature, objectionable characteristics of the conventional will be fullyexplained hereinafter.

The invention may be fully understod by refaudio volume control actionare eliminated, as

this invention.

Referring to the drawing, there is shown an intermediate frequencyamplifier IF such as commonly employed in the intermediate frequencystages of the'superheterodyne receiver and while only one such amplifieris shown, it will be understood that the receiving system may comprise aplurality-of intermediate frequency stages, as is well known. Theintermediate frequency amplifier shown is coupled to a detector D, whichconstitutes the usual second detector of the system, by means -of theinductively associated transformer coils P and S. The detector 1) iscoupled to .an audio-frequency amplifier AF in conventional manner, itbeing understood that there may be a plurality of audio-frequencyamplifier'stages as is well known. Automaticvol ume control may beembodied in the system in any manner, for example. by deriving a controlvoltage from the leak resistance of the detector D and applying suchvoltage through a filter resistor F to the control grid of amplifier IFinconventional manner. I

In the system shown, following the teachin of the above-mentionedcopending application, there is provided a, variable grid biasingresistor R which is interposed in the cathode lead of the intermediatefrequency amplifier and there is also provided a series resonant circuitA which includes a variable resistor R1, these two resistors beingpreferably adapted for joint or simuitaneous manual operation, asindicated by the broken-line representation. The circuit A also includesa condenser C and an inductance coil L which is overcoupled with respectto S. This device functions to increase the fidelity range of thereceiver, whilemaintaining the over-all gain uniform, as fully describedin the said copending application. Inasmuch as this subject matter doesnot form a part of cation, there is provided a branch circuit 13 whichis preferably connected to the main circuit of the intermediate stage IFand the detector D as illustrated. This circuit preferably takes theform of a sharply tuned circuit which has connected thereto a rectifierE preferably in the form of a grid-leak detector as illustrated. Unlessthe grid condenser 03 is so large that audio frequency waves will notoccur across it as a, result of demodulation ofthe carrier, other meansmust be li- 1 to prevent such'audlo frequencies from diathe presentinvention. it is unnecessary to describe it in further detail isdetected or rectified by the grid-leak detector E in the usual manner ofoperation of such a device. The output current of this detector. flowingthrough the indicator or meter M varies in accordance with the tuning ofthe receiver and the meter therefore indicates the proper tuning of thesystem. With the receiver adjusted for high fidelity reception, it isdifiicult to properly tune it by ear due to the fact that even thoughthereceiver is not properly tuned, reception will nevertheless takeplace but at a loss of quality. However, since the detector E is in asharply tuned circuit, it will be energized sufiiciently to operate themeter M only when the receiver is properly tuned. Thus the meterindicates at all times the correct tuning of the receiver, even when thecorrect tuning cannot be determined contenient- 1y by ear.

In accordance with the present invention, there is provided in theoutput circuit of the detector E and in series with 'the tuningindicator or meter M a resistor Raoneextremity of which is connected toa screen-grid g of, the audio frequency amplifier AF. There is alsoprovided a suitable filter condenser C2, as illustrated. By. virtue ofthe output current fiow through resistor R2, there is set up across thisresistor a unidirectionalpotential or voltage having the polarityindicated. It will be noted that the negative side of the resistor isconnected to the screen-grid g, of the audio frequency amplifier.

The cathode'ofone or more of the tubes AF may be biased above ground bya battery V have its cathode and as the amplification is roughly ing thepolarity indicated or this voltage may be supplied by a'circuit in theplate supply. It will be noted that the screen-grid-to-cathode circuitof this tube is shunted by the series circuit comprising the tuningmeter M and the internal 'plate-to-cathode circuit of the detector tubeE and that both of these ,tubes" are energized through the resistor R2.The condensers C1 and C: are preferably provided as filter condensersand serve to reduce the ripple in unidirectional current supplied to themeter M and screen-grid a. It will also be noted that when the grid ofthe detector tube E has no bias, corresponding to zero carrier signal,the'plate-to-cathode resistance of the tube E will be small. Hence themajor portion of the voltage dropin the circuit will be across R2. Byvirtue of the bias V on the cathode of tube AF, the screen-grid bias ofsaid tube may be very small or even negative with respect toproportional to the screen-grid bias for'low'bias values, the gain inthe amplifier. AF under these conditions 'may 'be reduced to zero; inother words, the tube may be cut off. But as the grid of the detectortube is made negative due to an increase in applied signal voltage, itsplate current will decrease and hence the voltage drop across theresistor R2 will decrease and the positive screen-grid voltage of theamplifier tube AF will increase. It will be noted that when the detectortube E is unbiased and consequently draws have little current, thenthe-screen voltage is determined largely by the current which the grid 9draws .per se and the relatively small plate current for the detectortube E will have little or no effect. Hence the circuit may be designedso that for zero or very small signals on the detector tube, theamplifier is substantially cut off; but for caused to operate as. aninter-carrier suppressor. Since it ischaracteristic of a grid-leakdetector that the output or plate current decreases with increase ofinput voltage, the output current of the detector E will be a minimumwhen the receiver is tuned to a carrier wave and a maximum when nosignal is applied. Therefore, the negative voltage across resistor Re,which is applied to the amplifier AF, will be a minimum when thereceiver is properly tuned to a carrier wave. With proper design, theamplifier AF may be caused .to transmit signals only when the signalinput to detector E is above a predetermined minimum value; or, in otherwords, when the receiver is properly tuned to a carrier wave. A decreasein detector input signal below this minimum value due to a change in thetuning of the receiver or-for any other reason, will cause theplate-to-cathode current of detector E to become sufilciently large tocausecut-oif or blocking of the amplifier AF so that the amplifier willnot function when the receiver is detuned. Thus, the

device may be made to function effectively as an inter-carriersuppressor.

. By properly 'designing the resistor R2 in rela-. tion to the amplifierAF, the device may thus be It will be noted that the present inventionhas I important advantages in connection with automatic volume control.The automatic volume con- Y trol action is controlled byenergy derivedfrom energy derived from portions or-circuits of the system whicharesharply resonant to the frequency of the received signals and,therefore, the suppressor cuts off the audio amplifier before the saidnoisy regions'can .be reached and while a large amount of automaticvolume control action is stillpresent, thereby eliminating theundesirable noisy operation. The suppressor action is at all timessharper than the automatic volume control action 'under any condition ofadjustment of the fidelity of response. The relatively broad frequencyresponse characteristics of the automatic volume control will maintainthe voltage supplied to the branch circuit B substantially constant overthe small range of frequencies to which the response of theinter-carrier noise sup pressor is limited by the sharply tuned inputcircuit of detector E. As the voltage supplied to it is not reduced bythe automatic volume control, the inter-canier noise suppressor is notcritical with respect to voltage.

Only such portion of the receiving system as is necessary to illustratea simple, embodiment of the invention has been illustrated, it beingunnecessary to illustrate or describe the conventional receiver infurther detail. It will be understood Edi . a oaaae L that variousmodifications, includingthose above a branch circuit comprising at leastone sharply tuned circuit energized by said intermediate frequencyamplifier, rectifying means energized by said intermediate frequencyamplifier for demodulating said modulated signal, an'audio frequencyamplifier comprising at least one space discharge device having ananode, a cathode and two control elements, means for supplying saiddemodulated signal to said cathode and one of said control elements, asecond rectifying means connected to said branch circuit and having anoutput circuit wherein there is obtained a signal dependent upon saidcarrier signal amplitude, and means including filter means for supplyingsaid last-named signal to said cathode and said other control element soas to reduce the amplification of said audio frequency amplifier whensaid carrier signal amplitude is less than a predetermined value;

' 2. In a superheterodyne radio receiver, an intermediate frequencyamplifier for amplifying modulated intermediate frequency carriersignals, a branch circuit comprising at least one sharply tunedcircuitenergized by said intermediate frequency amplifier, rectifyingmeans energized by said intermediate frequency amplifier fordemodulating said modulated signal, an audio frequency amplifiercomprising atleast' one space discharge device having an'anode, acathode and two control elements, means for supplying said demodulatedsignal to said cathode and one of said control elements, a secondrectifying means comprising a grid-leak detector having an inputcircuit, a cathode and an anode, means for supplying said detector inputcircuit with signals from said branch circuit, a resistance connected tosaid detector anode and to a voltage source, a connection between saiddetect-or cathode and said voltage source, a condenser connected to saiddetector anode and to said voltage source, a' conductive connectionbetween said detector anode and the other of said control elements ofsaid discharge device, and a conductive connection between said cathodeof said discharge device and said cathode of said grid-leak detector.-

3. In a superheterodyne radio receiver, an intermediate frequencyamplifier having a substantially rectangular frequency responsecharacteristic for amplifying modulated intermediate frequency carriersignals, automatic volume control means controlled by said intermediatefrequency amplifier, a branch circuit comprising at least one sharplytuned circuit energized by said intermediate frequency amplifier,rectifying means energized by said intermediate frequency amplifier fordemodulating said modulated signal, an audio frequency amplifiercomprising at least one space discharge device having an anode, a;cathode and two control elements, means for supplying said demodulatedsignal to said cathode and one of said control ele ments, a secondrectifying means connected to said branch circuit and having an outputcircuit wherein there is obtained a signal dependent upon said carriersignal amplitude, and .means' including filter means for supplying saidlast named signal to said cathode and said other control element so asto reduce the amplification of said audio frequency amplifier when saidcarrier signal amplitude is less than a predetermined .value.

4. In a superheterodyne radio receiver, an

device having an anode, a cathode and two con-' trol elements, means forsupplying said demodulated signal to said cathode and one of saidcontrol elements, a second rectifying means connected to said branchcircuit and having an output circuit wherein there is obtained a signaldependent upon said carrier signal amplitude, a tuning indicator in saidoutput circuit, and means including filter means for supplying said lastnamed signal to said cathode and said other control element so as toreduce the amplification of said audio frequency amplifier when saidcarrier signal amplitude is less than a predetermined value.

REYNOIDS 1). BROWN, in.

